Electrical resistance paint capable of forming a heating film



Api'il 21, 1 5 C. ORR, JR 2,883,307

ELECTRICAL RESISTANCE PAINT CAPABLE OF FORMING A HEATING FILM Filed July30, 1953 INVENTOR. CLYDE ORRMJR.

ATTORNEY United States Patent O ELECTRICAL RESISTANCE PAINT CAPABLE OFFORMING A HEATING FILM Clyde Orr, Jr., Atlanta, Ga., Tech ResearchInstitute, of Georgia assignor to The Georgia Atlanta, Ga., acorporation This invention relates to heating elements and moreparticularly to electrical resistance paint capable of forming a heatingfilm.

In the past, electrical resistance elements have been made by coating agraphite carrying silicate of soda on a carrying material, and theseelements have been used as resistors in radios and the like. Otherelectrical resistance coatings have been used for printed circuits andas elements carrying low currents or elements providing substantiallysmall quantities of heat at reasonably low temperatures. No resistancepaint, however, has been perfected heretofore which may besatisfactorily utilized to provide reasonably large quantities of heatat relativelyhigh temperatures.

In most instances, the prior art coating materials are first painted ona suitable carrier, then the carrier is inserted in an oven where theresistance paint is dried at controlled temperatures. In the baking ordrying process, the prior art resistance paints give 011? moisture whichtends to blister or break the continuity of the coating, therebyreducing its conductivity. Further, the baking process is time-consumingand requires additional equipment in order to provide a resistanceelement.

In substantially all instances, if a resistance element formed from aresistance paint, produced according to the prior art, were utilized asa heating element to provide substantial quantities of heat atreasonably high temperatures, the element would soon break down due to areaction between the conducting material and the oxygen in the air orthrough mechanical failure due to the formation of blisters. Thus it isapparent that the prior art resistance elements would have a reasonablyshort life; and, if substantial quantities of electricity were passedthrough these elements, the elements would gradually increase inresistance as the conducting material contained therein reacted with theair as described above.

Another disadvantage of the prior art resistance paints is that when thepaint is coated on a carrier, rapid fluctuations of temperature willcause cracking and a breakdown of the conducting surface due to thermalexpansion and contraction of the carrying element and resistance paint.

Accordingly, it is an object of my invention to provide a resistanceelement capable of contracting and expanding with rapid changes intemperature without appreciable change in resistivity thereof andwithout cracking.

Another object of my invention is to provide a resistance paint whichmay be coated on a carrying material and which will dry to become aserviceable heating element without additional treatment.

Another object of my invention is to provide a resistance paint whichwill adhere to a variety of materials such as ceramics, glass, mica,fiber and the like, to provide a resistance element which is not easilyseparated from the carrying material.

Another object of my invention is to provide a re sistance paint whichis capable of being stored in a liquid state for extended periods oftime without appreciable oxidation, skinning or livering of the surfacethereof.

scribing flask 10 and contacting all electrodes.

Another object of my invention is to provide a resistance paint whichwhen coated on a suitable carrying element will provide a substantiallyuniform, hard, flexible surface that does not blister on drying and inuse does not change its resistivity or color.

Another object of my invention is to provide a resistance element whichis capable of conducting electricity forextended periods of time atelevated temperatures without appreciable change in conductivity andappearance.

Other and further objects and advantages of my invention will becomeapparent from the following description when taken in conjunction withthe accompanying drawings, wherein like characters of referencedesignate corresponding parts through the several views, and wherein:

Fig. 1 is a side elevation showing my resistance paint coated on acarrying element, such as a Florentine flask.

Fig. 2 is a bottom plan view of the Florentine flask illustrated in Fig.1.

Referring now in detail to the embodiment chosen for purpose ofillustration, numeral 10 denotes a carrying element, such as aFlorentine flask, normally used in laboratories for heating solutionsand the like. Around the body portion of flask 10 are provided aplurality of spaced electrodes 11 which comprise wires circumscribingthe body of flask 10 with portions of each projecting outwardly to formterminals 12 to which an external source of electricity may beconnected. Any number of electrodes 11 may be placed between the twoextreme electrodes to provide graduated resistances to be described inmore detail later.

To form a heating element, resistance paint 13 is coated uniformlyaround the body of flask 10, between the extremes of electrodes 11, sothat it forms a coat circum- After resistance paint 13 has been coatedon flask 10, the resistance paint should be allowed to dry in air;however, the resistance paint may be dried by the passage of electricalcurrent therethrough.

In the event that the carrying element is an electrical conductor, itmay be found desirable to coat the carrying element with a protective orinsulating coating, and in most instances it will be found desirable tocoat the outer surface of the resistance paint with such a protectivecoating. In the drawings, numeral 14 denotes such a protective coatingwhich is painted or coated over the resistance paint, after suflicienttime has elapsed for the resistance paint to dry.

While I have selected a Florentine flask formed of glass, for purpose ofillustration, it will be apparent to those skilled in the art thatsubstantially any surface, to which my resistance paint adheres, may beutilized as a carrying element, regardless of its shape; and myinvention contemplates the use of my resistance paint on such surfacesas glass, mica, plastics, ceramics, and the like. It will be found thatmy resistance paint adheres easily to substantially any surface to whicha normal paint will adhere and is sufficiently flexible that thecoeflicient of expansion of the specific material is not to beconsidered a determining factor in the selection of a carrying element.

Protective coatings formed according to my invention have been found toadhere to such widely varying materials as glass and copper. Further,the inclusion of a protective coating over the resistance paint, hasmaterially increased the operable temperature of my resistance paint.

In utilizing my resistance paint and protective coating, each isseparately compounded as hereinafter, described. The resistance paint isthen coated on a grease-free electrically non-conducting carryingelement so as to form a continuous path between a pair of electrodeswhich are located to include the region to be heated. Copper wires orstrips are recommended for these electrodes. After the resistance paintis applied to the carrying element, the paint is dried. Another methodof drying my resistance paint is to heat it in an oven at 104 C. forthirty minutes; then the temperature may be raised to about 130 and thepaint dried for another thirty minutes.

After the resistance paint is dry, a protective coating may be appliedto the surface of the resistance paint, and it may be dried in a manneridentical to the drying of the resistance paint. Two or more protectivecoatings may be applied to the resistance paint, each coating beingdried separately.

It is to be remembered that the oven drying process described above isnot necessary to the practice of my invention but may be utilized todecrease the time necessary to prepare a heating element. Whenelectricity is first applied to the resistanceelement after it is dry,it is suggested that a variable transformer be utilized and that currentbe applied to the resistance element so that the temperature does notincrease more than about per minute. This precaution upon initialheating permits gases and vapors to escape without disrupting the filmand prevents the development of excessive strains which might causecracking or breakage of the carrying element or the resistance paint orboth.

A film of conducting compound formed from resistance paint havingconsistency of ordinary house paint, and covering a surface of one inchsquare, offers a resistance of the order of 30 ohms across oppositeedges. The resistance figure of course is not constant since variationin the thickness of film and changes in the quantity of conductivematerial within the resistance paint will alter this figure.

To utilize the apparatus heretofore described, current from an externalsource (not shown) is supplied to any two of electrodes ll, it beingobvious that in some instances it may be found desirable to supplycurrent to more than two of the electrodes 1. The temperature andquantity of heat supplied by the electricity may be determined by thecurrent passed through my resistance paint according to prior artcalculations. Because of the uniform surface and thickness which myresistance paint provides, the space between any two electrodes ll willdetermine the relative resistivity of that component part subtendedbetween those two electrodes; thus it is apparent that differentresistances of the resistance element may be selected by applyingcurrent to different combinations of electrodes 11.

In compounding resistance paint 13, according to my invention, anelectrically conductive powder is mixed with a binder or vehicletogether with certain modifying materials which impart to this mixturedesirable characteristics. is finely divided or in powdered form may beused to form an electrical conductor which may be evenly dis persedthroughout the binder or vehicle. Such materials as carbon, graphite,silver, platinum, gold, copper, nickel, and the like, in finely dividedform, have been found satisfactory for this purpose. Under certaincircumstances, aluminum may be used; however, since a non-conductingoxide coating is generally present on each aluminum particle, giving thealuminum a very low electrical conductivity, other materials, asdescribed above, may be found more desirable.

One or a plurality of the conducting materials described above areincorporated with a binder such as sodium or potassium silicate, watersolutions of magnesium chloride, phosphoric acid cements,polytetrafluoroethylene, and other similar materials which willwithstand the required temperature. Generally, I have found thatpotassium silicate and graphite are preferable in gredients for forminga resistant paint according to my invention.

Substantially any conducting material which i To impart flexibility to aresulting film formed by my resistance paint, and to retardsedimentation of the resistance paint when the paint is stored in afiuid condition for extended periods of time, bentonite is included inthe ingredients as a modifying ingredient. Since bentonite gels arethixotropic, the ability of my resistance paint to be formed in films ofdifierent thickness is enhanced. Further, the bentonite makes the filmresulting from my resistance paint somewhat porous, for purposes to bedescribed in more detail hereinafter. The same but poorer structuralmodifications may be achieved by the substitution of diatomaceous earthfor the bentonite. One diatomaceous earth material which has been foundsuit able is known by the trade name Celite, a product of TheJohns-Manville Company, New York, New York. Fullers earth or clay mayalso be substituted for the bentonite in the formation of a resistancepaint according to my invention.

Another modifying ingredient which serves to keep the conductingmaterial in colloidal suspension while the paint is in liquid form andalso serves appreciably to reduce the reaction between such binders assodium silicate and the carbon dioxide of the air, is a finely dividedcarbonaceous material such as coal or bitumen. When this carbonaceousmaterial is incorporated in the compounding of my resistance paint, ittends to react with the alkaline silicate causing the formation of smallbubbles within the liquid resistance paint after the carbonaceousmaterial is introduced therein. The carbonaceous material also servesanother highly important function; namely, when the resistance paint isheated to high operating temperatures, the carbonaceous materialdecomposes, thereby depositing carbon in the resistance paint tosupplement some of the conducting material which may have been burnedout at high temperature. In addition, the carbonaceous material tends toreact with the free oxygen entrained in the resistance paint much morereadily than the electrical conductor, and thus it will leach outentrained oxygen within the paint. Further, upon the decomposition ofthe carbonaceous material, tar will be deposited within the resistancepaint and acts as an additive binder which tends to choke out entrainedgaseous particles. The carbonaceous material, such as coal, decreasesthe eflfervescence of sodium silicate and thus retards the formation ofa chalk-like appearance which usually appears on the surface ofresistance paint film containing sodium silicate. The reaction of thecarbonaceous material with the alkali silicate also tends to facilitatethe deposition of silica, thus adding to the stability of the resistancepaint when dry and decreasing the alkalinity of the resistance paint.

A resistance paint according to my invention may be formed of thefollowing materials in substantially the following proportions:

Percent by weight Sodium or potassium silicate (silicate having anapproximate alkaline silica ratio of 1:3

and a density of from 40 to 50 Baum) 30 to Graphite (finely divided) 10to 30 Bituminous coal (finely divided) 5 to 1.0 Bentonite (finelydivided) 1 to 15 Water (in addition to that in the silicate) 0 to 50 Incompounding my resistance paint, the bentonite should be commingled withthe other ingredients as a bentonite gel. This gel is formed by mixingnot more than about 18% by weight bentonite with water for a period ofseveral hours. Except for the bentonite, the other materials may bemixed in any order, and for a sufiicient length of time to providethorough commingling of the ingredients. The resulting material developsits optimum properties two to three days after being mixed. Water isincluded as an ingredient in my resistance paint so that the resultingliquid may have the consistency of ordinary house paint. It is to beremembered that the Example I G. Silicate of soda (alkali-silica ratio1:2.90) 20 Water 8 Acheson graphite (a minimum of 98.5 percent by weightpasses a ZOO-mesh screen) 8 Coal (100% thru a 325-mesh screen) Bentonitegel (17.6% by weight bentonite) 2.5

Example II Potassium silicate (Kasil No. 6 of Philadelphia Quartz Co.)20 Water, to give consistency of paint. Graphite (natural product ofJoseph Dixon Co., No.

1107) Coal 100% thru a 325-mesh screen) 1.5 Water and bentonite gel(containing 17.6 percent by weight bentonite) 1.5

Example III Potassium silicate (Kasil No. 6 of Philadelphia.

Quartz Co.) 20 Acheson graphite (a minimum of 98.5% by weight passes aZOO-mesh screen) 7.5 Water, to give consistency of paint. Coal (100%thru a 325-mesh screen) 2 Bentonite gel (containing 17.6 percent byweight bentonite) 2 Example IV Potassium silicate (Kasil No. 6,Philadelphia Quartz Co.) 20 Acheson graphite (a minimum of 98.5% passesa ZOO-mesh screen. Product of National Carbon Co., Buffalo, NY.) 7.5Coal (100% thru 325-mesh screen) 5 Bentonite gel (17.6 percent by weightbentonite) 2 Water, to give consistency of paint.

Example V Sodium silicate (K brand of Philadelphia Quartz Co.) 31.2Colloidal graphite (from Acheson Colloids Co., and having a maximumparticle size with few exceptions) of 4 microns 6 Water 18.6 Bentonitegel (17.6 percent by weight bentonite) 2 Coal 100% thru 325-mesh screen)2 The formation of a resistance paint utilizing the ingredients ofExample IV, has been found to be most generally useful, although itssuperiority over the other examples is small. Because of the expense ofpotassium silicate and because the inclusion of coal or bitumen, byreducing the reaction between sodium silicate and the carbon dioxide ofthe air as described herein, makes sodium silicate acceptable,formulations employing sodium silicate would probably be more practicalfrom an economic standpoint.

Two specimens of resistance paint formulated according to Example Ihereof were tested by coating a one inch square film on a flat sheet ofglass while maintaining the temperature at a substantially constantfigure. The graphs below illustrate the resistance of this film on aday-by-day test.

\/ Heating at 500 F.

Eleetrical resistance, ohms/in. of film llllllllllllll 200 400 000 8001000 1200 1400 Time of heating, hrs.

As described above, a protective or insulating coating or film may beincluded in the formation of a heating element, according to myinvention, and it is to be remembered that when coating on a conductingsurface, the protective film must be applied to the surface of thematerial prior to the application of the resistance paint.

A suitable protective film may comprise the following:

Weight percent (1) Sodium or potassium silicate (silicate having anapproximate alkali-silica ratio of 1:3 and a density of 40 to degreesBaum) 30 to (2) Alumina (finely divided) 25 to 35 (3) Aluminum (finelydivided) 2 to 5 (4) Bentonite (finely divided) 0.5 to 10 (5) Water (inaddition to that in the silicate) 0 to 50 As described above, thebentonite of the protective coating is made up as a bentonite gel and isthen thoroughly commingled with the other ingredients.

Other examples of the protective films are as follows:

Alumina (for abrasive purposes, about 4 microns in diameter or less ofthe Linde Air Products Co., lot No. P-52, Type A5175) 8.7

Aluminum (pigment, Albron, No. 322 of Aluminum Co. of America) 2.9 Water16.6

Example Ill Sauereisen cement (Paste No. l of the Sauereisen CementsCo., Pittsburgh, Pa.) 20

Silicate of soda (N Brand of the Philadelphia Aluminum (pigment, Albron,No. 322 of Aluminum Co. of America) 4 Water, to give consistency ofpaint.

assaao'r The aluminum in each compound gives flexibility so thatexpansion and contraction will not disrupt the films. The potassiumdichromate probably inhibts reaction between the alkali and thealuminum. Since a protective oxide film exists about each aluminumparticle; the aluminum does not significantly increase the electricalconductivity of this protective paint.

It is obvious to those'skilled in the art that many variations may bemade in the embodiments chosen for purpose of illustration withoutdeparting from the scope of my invention as defined by the appendedclaims.

I claim:

1. An electrically resisting reaction product film resulting from thedrying and heating to a temperature up to 500 F. of a conducting paintapplied in a homogeneous layer to a surface, said conducting paintconsisting essentially of by weight approximately 30% to approximately80% a silicate water solution having an initial density equivalent tofrom about 40 to about 50 Baum and selected from the group consisting ofsodium silicate water solution and potassium silicate water solution,from approximately 10% to approximately 30% a finely divided conductingmaterial, from approximately 5% to approximately 10% a carbonaecousmaterial selected from the group consisting of coal and bitumen, fromapproximately 1% to approximately 15% a modifying agent selected fromthe group consisting of ben tonite, fullers earth clay and diatomaceousearth and up to approximately 50% water.

2. An electrically resisting film resulting from a reaction caused whensufficient electricity is passed through a conducting paint to heat thesame, said conducting paint initially being applied in a homogeneouslayer to a surface for the manufacture of heating elements comprisingmixture from approximately 30% to approximately 80% by weight a silicatewater solution having an initial density equivalent to from about 40 toabout 50 Baum and selected from the group consisting of sodium silicatewater solution and potassium silicate water solution, from approximately10% to approximately 30% by weight graphite, from approximately 5% toapproximately 10% by weight coal, from approximately 1% to approximately15% by weight bentonite and from to approximately 50% by weight water.

3. A free flowing electrically conducting paint adapted to be applied ina homogeneous layer to a surface for the manufacture of heating elementsconsisting essentially of proportions approximately equivalent to 20grams of silicate of soda, 8 grams of water, 8 grams of graphite, gramsof coal and 2.5 grams of bentonite gel.

4. A free flowing electrically conducting paint adapted to be applied ina homogeneous layer to a surface for the manufacture of heating elementsconsisting essentially of proportions approximately equivalent to 20grams of potassium silicate, 10 grams of natural graphite, 1.5 grams ofcoal, 1.5 grams of bentonite gel and water to give the consistency ofpaint.

5. A free flowing electrically conducting paint adapted to be applied ina homogeneous layer to a surface for the manufacture of heating elementsconsisting essentially of proportions approximately equivalent to 20grams of potassium silicate, 7 .5 grams of graphite, 2 grams of coal,Zgrams of be'ntonite :gel, and water to give the consistency of paint.

6. A free flowing electrically conducting paint adapted to be applied ina;homogeneous layer to a surface for the manufacture of heating elementsconsisting essentially of proportions approximately equivalent to 20grams of potassium silicate, 7.5 grams of graphite, 5 grams of coal, 2.0grams of bento'nite gel and water to give the consistencyof paint.

7. A free flowing electrically conducting paint adapted to be applied ina homogeneous layer to a surface for the manufacture of heating elementsconsisting essentially of proportions aproxi'ma'tely equivalent to 31.2grams of sodium silicate, 8.6 grams of water, 6 grams of colloidalgraphite, 2 grams of coal and 2 grams of bentonite gel.

8. A method of making a resistance element for heating purposes, whichincludes mixing from approximately 30% to approximately by weight asilicate water solution selected from the group consisting of sodiumsilicate and potassium silicate, from approximately 10% to approximately30% by weight a finely divided conducting material from approximately 5%to approximately 10% by weight coal, from approximately 1% toapproximately 15 by weight bentonite and up to approximately 50% byweight water to make a free flowing resistance paint, coating aninsulation carrying element with a thin film of said resistance paintand drying said film in air.

9. A method as claimed in claim 8 wherein an electrically insulatingfilm is coated over said film of said resistance paint after said filmof said resistance paint has dried.

10. A protective coating material for electrical resistance elementscomprising by weight from approximately 30% to approximately 80% asilicate selected from the group consisting of sodium silicate andpotassium silicate, from approximately 25% to approximately 35% alumina,from approximately .5% to approximately 10% bentonite and up toapproximately 50% water.

References Cited in the file of this patent UNITED STATES PATENTS617,375 Voigt et al. Ian. 10, 1899 798,260 Bristol Aug. 29, 19051,762,990 Jones June 10, 1930 2,047,087 Szymanowitz July 7, 19362,261,260 Kraus Nov. 4, 1941 2,330,782 Morelock Sept. 28, 1943 2,409,893Pendleton Oct. 22, 1946 2,479,914 Drugman et al. Aug. 23, 1949 2,683,673Silversher July 13, 1951 2,703,356 Buchanan Mar. 1, 1955 OTHERREFERENCES Printed Circuit Techniques, National Bureau of StandardsCircular #468, November 15, 1947, 117 RC. Pages 6 and 19.

8. A METHOD OF MAKING A RESISTANCE ELEMENT FOR HEATING PURPOSES, WHICHINCLUDES MIXING FROM APPROXIMATELY 30% TO APPROXIMATELY 80% BY WEIGHT ASILICATE WATER SOLUTION SELECTED FROM THE GROUP CONSISTING OF SODIUMSILICATE AND POTASSIUM SILICATE, FROM APPROXIMATELY 10% TO APPROXIMATELY30% BY WEIGHT A FINELY DIVIDED CONDUCTING MATERIAL FROM APPROXIMATELY 5%TO APPROXIMATELY 10% BY WEIGHT COAL, FROM APPROXIMATELY 1% TOAPPROXIMATELY 15% BY WEIGHT BENTONITE AND UP TO APPROXMATELY 50% BYWEIGHT WATER TO MAKE A FREE FLOWING RESISTANCE PAINT, COATING ANINSULATION CARRYING ELEMENT WITH A THIN FILM OF SAID RESISTANCE PAINTAND DRYING SAID FILM IN AIR.
 9. A METHOD AS CLAIMED IN CLAIM 8 WHEREINAN ELECTRICALLY INSULATING FILM IS COATED OVER SAID FILM OF SAIDRESISTANCE PAINT AFTER SAID FILM OF SAID RESISTANCE PAINT HAS DRIED.